06. Project Summary/Abstract This application raises the possibility that the myocyte compartment of the embryonic, fetal and post-natal heart is generated by activation and lineage commitment of a pool of resident c-kit-positive cardiac progenitor cells (CPCs) which are clustered in niches within the primitive heart. Cardiac morphogenesis may be mediated by spontaneous calcium oscillations within CPCs which lead to cell growth and the acquisition of the myocyte phenotype. The possibility is raised that the fate of CPCs is regulated by transient changes in intracellular calcium which constitute the essential element of symmetric and asymmetric division of these primitive cells. Similarly, calcium oscillations condition the differentiation of CPCs into functionally competent myocytes which become electrically and mechanically excitable. To address these fundamental issues, two transgenic mouse models have been developed: one in which EGFP is under the control of the c-kit-promoter (c-kit-EGFP mouse) and the second in which EGFP expression is regulated by the cardiac specific -myosin heavy chain promoter (MHC-EGFP mouse). The c-kit-EGFP mouse should allow us to identify the embryonic stages at which c-kit-positive-EGFP-positive CPCs appear in the forming heart, their anatomical distribution and developmental changes in prenatal and postnatal life. The MHC-EGFP mouse will permit us to define the localization and spatial distribution of forming myocytes postnatally and this information will be complemented with the data to be obtained in the c-kit-EGFP mouse. With these two models, the relationship between the generation of myocytes and the activation, commitment and differentiation of CPCs will be established. These studies will be integrated with the analysis of the electrophysiological, mechanical and calcium handling properties of CPCs and linearly related cells together with their pattern of growth and differentiation. Ultimately, the interdependence of cellular physiology and growth with calcium being the master regulatory system will be determined. Therefore, the role that intracardiac progenitor cells have in the developing heart will be characterized and this information may have important implications in the myocardial adaptations to ischemic and non-ischemic damage later in life.